Boron nitride (BN) is commercially available and is relatively inexpensive as a powder. Practical applications of BN, however, require that the BN be in the form of a coating, fiber or monolith rather than as a powder. Although coatings can be derived from the powder by physical vapor deposition, processes for converting BN powder into fibers are not available currently and processes for transforming powder into monoliths are energy and equipment intensive. Accordingly, there is a need to provide an effective manner of providing BN in forms other than as a powder.
A possible way of doing this is to use tractable "preceramic" BN precursors that can be manipulated into the desired form and then pyrolyzed to BN. Polymeric amine/boron compounds such as polyaminoboranes (PABs) and polyborazines (PBZs), representative structures of which are set forth below, are potential precursors to BN. ##STR1## Me=methyl, x represents a positive integer
In fact, PABs and PBZs have been shown to give BN on pyrolysis (Inorg. Chem. (1963) 2: 29). There is not presently available, however, a selective, high yield synthetic route for making nonvolatile, but tractable PABs and PBZs that can be converted to high yield ceramics.
In current low temperature amine/borane chemistry, Lewis acid-Lewis base interactions dominate almost all of the known reaction chemistry of amines with boranes. The strength of the Lewis acid-Lewis base bond drives aminoboranes to form volatile tricyclomeric complexes or intractable polymers. Low temperature amine/borane chemistry that involves such interactions usually do not provide a feasible synthetic route to tractable oligomeric or polymeric BN precursors.
Other synthetic routes that are currently available to produce tractable PABs and PBZs rely on heating the reactants to high temperatures to obtain condensation-like products. Heating the aminoborane complex NH.sub.3.BH.sub.3 to its decomposition point, for instance, provides mixtures of condensed oligoborazines with limited solubilities in common organic solvents. If this reaction is run above 200.degree. C., both borazine and insoluble condensed PBZs are produced (Laubengayer, A. W., et al, J Am Chem Soc (1961) 83: 1337). On heating, these insoluble PBZs decompose at 900.degree. C. to give moderate yields of BN. Similar reactions using substituted aminoboranes, lower temperatures and ammonium chloride as a catalyst yield substituted borazine and oligomeric compounds (Hawthorne, J., J Am Chem Soc (1959) 81: 5836 and J Am Chem Soc (1961) 83: 833). When borazine is heated to temperatures of 340.degree. C. to 380.degree. C. it rearranges to form higher aromatic-like compounds (Laubengayer, A. W., et al, supra). These condensed aromatic-like compounds can be converted to BN by pyrolysis; unfortunately, however, many of them are intractable solids or too volatile which makes them unattractive as preceramic materials.
Two alternative approaches to the synthesis of PABs and PBZs derive from the condensation reactions of trisaminoboranes (Aubrey, D. W. and Lappert, M. F., J Chem Soc (1959) 2927; and Burch, J. E., et al, J Chem Soc (1962) 2200). These reactions provide soluble cross-linked elastomers; but, the reaction temperatures are again quite high and this significantly affects yields and selectivities.
U.S. Pat. No. 2,809,171 discloses that linear PABs are produced by heating isopropylaminoborane at 300.degree. C. This approach to tractable PABs works but the presence of the relatively large isopropyl group will lead to low ceramic yields. Also, as indicated, the synthesis requires relatively high reaction temperatures.
There are three reports that describe the synthesis of tractable PBZs used as ceramic precursors. Japanese Kokai Patent Publication 76/53,000 (1976) describes the preparation of preceramic PBZs by the following reaction: ##STR2## Ph=phenyl The polymer produced by this reaction was successfully converted into a preceramic fiber and thence to a BN fiber by pyrolysis. Again, however, this synthesis requires high temperatures and most likely suffers from lack of selectivity, and low ceramic yields.
The second report (Narula, C. K., et al, "Better Ceramics Through Chemistry" Symposium, Mat Res Soc (1986) in press) describes the preparation of methyl-substituted PBZs at low temperatures by the following scheme: ##STR3## Pyrolysis of these polymers provides ceramic yields of &lt;50%. The ceramic products are not completely characterized but are low in nitrogen and contain some silicon. Some of the reported problems using this approach include difficulties with solvent removal at low temperatures, incomplete reaction of all the substituents as evidenced by the evolution of ammonium chloride during pyrolysis and the fact that the precursors are usually gels that are not suitable for spinning.
The third report involves unpublished attempts to synthesize PBZs using silicon chloride elimination reactions of the following type: EQU (Me.sub.3 SiNH).sub.2 BCl.fwdarw.Me.sub.3 SiCl+[(Me.sub.3 SiNH)BNH].sub.3 +[(Me.sub.3 SiNH)BNH].sub.x
While these polymers do produce BN upon pyrolysis, the ceramic yields are on the order of 20%-30% because the trimethylsilyl ligands must be lost in the conversion process.
Summarizing the current state of the art relating to synthesizing BN preceramic polymers leads to the following conclusions:
1. The low-temperature chemistry of aminoboranes is dominated by their Lewis acid-Lewis base interactions which limits synthetic routes to tractable polymers.
2. Those synthetic routes that are available require relatively high temperatures which leads to poor selectivity and low overall yields of tractable precursors.
3. Tractable BN preceramic polymers currently available contain high weight percents of extraneous side groups (e.g. isopropyl and phenyl groups) that lead to low ceramic yields.
A primary object of the present invention is to provide a novel low temperature catalytic synthesis of BN precursors, precursors to compounds of boron/other Group VA nonmetals, and precursors for other Group IIIA metal/Group VA nonmetal compounds.
In addition to being preceramic materials, compounds containing B-N bonds may be useful as synthetic reagents. For example, tris(dimethylamino)borane is a useful reagent for boron templated cyclization in total synthesis of monocyclic spermidine alkaloids. Group IIIA metal-Group VA nonmetal ceramics other than BN have various commercial applications. For instance, AlN is a high refractory and chemical resistant with important physical and electronic properties and GaAs offers semiconductor properties not available with Si-based electronic materials.